There are several ways a floating structure may be hold steady in relation to a point at the seabed. It may be anchored with inclined anchor lines or vertical anchor lines (as a tension leg platform) or it may be dynamically positioned. In all these different methods will the vessel or platform undergo some movements vertically and horizontally due to waves, wind currents or similar. For all these methods there would be set limits for how much the vessel or platform is allowed to move vertically and horizontally, but there will always be some dynamics in a system with a riser between a point at seabed and a floating vessel or platform, and there are several ways to handle this dynamics.
For a floating structure that is vertically anchored (a tension leg platform) so that the length of the risers is more or less constant, metal risers may be employed that are straight and vertical. Even if the floating structure is a tension leg platform there will be some movement and the risers are normally equipped with heave compensators on the platform deck to compensate for small changes in length and stiffness. Generally there is always a wish for reducing the amount of equipment on a vessel or platform, due to limitations in weight and space. The riser is also usually equipped with stress joints at the seabed. Such stress joint are lengths of tapered pipe. Since stress joints scale to some power of the diameter, they become very large as the diameter is increased, and this imposes practical limits on their maximum diameter.
For vessels or platforms that use inclined anchor lines or are dynamically positioned, the distance between the riser's end point on the vessel and on the seabed may vary considerably due to alterations in the vessel's draught, tides, wind and waves, or as a result of damage to the vessel or the anchor system. In such cases flexible hoses are commonly used, often equipped with buoyancy and ballast to increase their flexibility. Flexible hoses are expensive and there is a wish for using metal risers.
The simplest form is a J-shape, where the riser is in the form of a catenary from the tangential point on the seabed to the platform. This is only suitable for applications where the water depth is several times the maximum horizontal platform movement and where the dynamic platform motions are limited.
A more common form is that of a reclining “S”, where the weight of the hose makes it concave up near the end that is connected to the platform, and buoyancy elements make it concave down near the end that is connected to the seabed. From here a continuation resting on the seabed leads to an installation at the seabed. The riser is kept taut by one or two anchor ropes fastened to an anchor. The total length of this riser configuration is approximately 3 times the water depth, and the radii of curvature are so small that the pipe has to be in the form of a flexible hose. In an attempt to use titanium, which can withstand substantially smaller bending radii than steel, it was found that the pipes had to be bent to nearly their final shape, which resulted in considerable installation problems.
One possible solution for a riser configuration with rigid riser elements is a riser as described in WO 97/21017. The riser between the connection point at the seabed and the floating platform, consists of two rigid elements connected with a weighted bend in an angle of more or less 90 degrees near the seabed. This configuration, however, allows for only small movements of the floating structure in a horizontal plane. This is so because the weighted bend always will tend to keep the riser part between the bend and the floating platform in a vertical position and this will give unwanted and critical forces in the substantially horizontal part of riser.
The object of the present invention is to replace these known arrangements with one that allows a shorter riser and which riser does not require buoyancy elements, while at the same time having large flexibility in relation to movements of the floating structure. Another object is to achieve a riser consisting mainly of straight pipe elements, and which is of such a nature that the limited flexibility of metal (steel or titanium) is adequate. A further object of the invention is to produce a riser system with large flexibility in relation to movements of the floating structure which at the same time does not use much space on the seabed.